Thorium, which is an actinide element, is usually accompanied with rare earth elements. Tributylphosphate (TBP) was used as an extractant to extract and separate thorium from nitrate medium in the early process of separating thorium. However, TBP is consumed in large amount due to the high water solubility thereof. In addition, the extraction process should be performed in a medium containing a high concentration of nitric acid, which causes consumption of a large amount of nitric acid and accelerates the decomposition of TBP.
Currently, N1923, which is a primary amine, is predominantly used to extract and separate thorium from a sulfate medium, while di(2-ethylhexyl)-2-ethylhexylphosphonate (DEHEHP) and the like may be used to extract and separate thorium from a nitric acid system. For example, Chinese patents ZL98122348.6 and ZL02123913.4 disclose processes of extracting and separating thorium from the leaching of bastnaesite, or mixed bastnaesite-monazite ore, in which the primary amine N1923 is used as the extractant to extract and separate thorium, and thorium in the leaching may be recovered and enriched with a purity of 95%˜99% and a recovery of 95%. In addition, acidic phosphorus (phosphine) extractants, such as P204 and P507, have good extractive performance for thorium, but the stripping of the loaded thorium is hard so that these acidic extractants are rarely applied in industries.
The present inventors have been studying the separation and purification of thorium and rare earth elements for a long term, and were granted two Chinese invention patents, ZL201110074345.8 and ZL201210552752.X, in the separation and purification of thorium. The extractants used in these two patents are both a neutral phosphine extractant with a formula of G3P═O, wherein each G is independently selected from the group consisting of alkyl and alkoxy. However, there are still some disadvantages such as non-easily obtained raw material, high cost and limit to use in a special medium.
Thus, there is a need to develop novel processes of extracting and separating thorium, and search for novel extraction systems.